Power Plant Engineering

94 POWER PLANT ENGINEERING

Hybrid Geothermal Power Plant-Fossil System. The concept of hybrid geothermal-fossil-
fuel systems utilizes the relatively low-tem-perature heat of geothermal sources in the low-temperature
end of a conventional cycle and the high-temperature heat from fossil-fuel combustion in the high-
temperature end of that cycle. The concept thus combines the high-efficiency of a high-temperature
cycle with a natural source of heat for part of the heat addition, thus reducing the consumption of the
expensive and nonrenewable fossil fuel.

There are two possible arrangements for hybrid plants. These are
(1) Geothermal preheat, suitable for low-temperature liquid-dominated systems, and
(2) Fossil superheat, suitable for vapor-dominated and high-temperature liquid-dominated
systems.

Geothermal-Preheat Hydrid Systems. In these systems the low-temperature geothermal en-
ergy is used for feed water heating of an otherwise conventional fossil-fueled steam plant. Geothermal
heat replaces some, or all, of the feed water heaters, depending upon its temperature. A cycle operating
on this principle is illustrated in Fig. 2.37. As shown, geothermal heat heats the feed water throughout
the low-temperature end prior to an open-type deaerating heater. The DA is followed by a boiler feed
pump and three closed-type feed water heaters with drains cascaded backward. These receive heat
from steam bled from higher-pressure stages of the turbine. No steam is bled from the lower-pressure
stages because geothermal brine fulfills this function.

High-pressureturbine

Low

pressure

turbine

Surface

Condenser

Condenser

pump

Hot

brine

Cold

brine

Geothermal

heat exchanger

Deaerating

heater

Boiler

feed pump

Feedwater heaters

Superheater

Reheater

Evaporator

Economizer

Fig 2.37. Schematic of a Geothermal-Preheat Hybrid System.
Fossil-Superheat Hybrid Systems. In these systems, the vapor-dominated steam, or the vapor
obtained from a flash separator in a high-temperature liquid-dominated system, is superheated in a
fossil fired super heater.

Fig. 2.38 show schematic flow. It comprises a double-flash geothermal steam system. Steam
pro-duced at 4 in the first-stage flash separator is preheated from 4 to 5 in a regenerator by exhaust
steam from the high-pressure turbine at 7. It is then superheated by a fossil fuel fired super heater to 6
and expands in the high pressure turbine to 7 at a pressure near that of the second stage steam separator.
It than enters the regenerator, leaves it at 8, where it mixes with the lower pressure steam produces in
the second stage flash separator at 15, and produces steam at 9, which expands in the lower pressure